It is known that use of manganese dioxide as an active cathode material in a rechargeable alkaline secondary cell is optimized if the discharge capacity is limited to one electron-reduction of manganese dioxide. If the reduction proceeds beyond the one electron transfer, that portion of the manganese compound, which so proceeds, cannot be recharged. This is particularly true in the presence of highly conductive alkaline electrolytes.
If such a rechargeable cell were provided with a zinc anode having an electrochemical capacity equal to or higher than the one electron discharge capacity of the manganese dioxide cathode, then discharge of the cell would have to be terminated at the end of the one-electron reduction, in order to preserve efficient rechargeability of the cell.
To overcome the problems discussed above, the practice of "zinc limitation" is now known. Using zinc limitation, the quantity of zinc used in the anode has a pre-determined oxidation capacity equal to or less than the corresponding reduction capacity of the active manganese dioxide in the cathode at the single electron limit. The respective reaction "rate" capacities are, of course, separate and distinct parameters, not in general addressed here.
Thus, when the zinc reaction capacity is exhausted, even though there may be active manganese dioxide left in the cathode, the cell cannot be discharged further, of its own power, because the reaction capacity of the zinc anode has been depleted. The voltage of a single such cell goes to approximately zero, if it remains connected to a load.
However, if the cell is connected in series with at least one additional cell which still has some ampere-hour capacity left, then the exhausted cell can be subjected to reverse polarity (also referred to herein as Series Reversal) if the cell remains connected to the load and in series with the other cell. Namely, the one or more cells which are still active force current to pass through the exhausted cell, thus creating the reverse polarity.
Reversing polarity in a primary (disposable) cell under such conditions is of lesser consequence because the polarity reversal occurs after the cell has been fully discharged and therefore its useful life has been exhausted. Reversing polarity in a secondary (rechargeable) cell, however, has real consequences because, after the zinc has been depleted in a secondary cell, detrimental irreversible electrochemical changes occur in the cell. The mere fact that a secondary cell has been fully discharged does not mean that its cycle life has been exhausted. Rather, the cell will typically be recharged, and discharged to produce useful work again, and again.
Since a rechargeable cell is intended to be cycled through several, preferably many, discharge/recharge cycles during its useful life, such irreversible chemical changes can greatly reduce the useful life of the cell. Thus, in a rechargeable cell, it is important to minimize, preferably eliminate or avoid, irreversible chemical reactions. Rather, all chemical reactions should preferably be completely reversible. The higher the degree of reversibility the greater the prospect for extended cell life.
It is an object of this invention to provide a secondary alkaline zinc-manganese dioxide cell having a novel anode current collector with reduced susceptibility to oxidation, especially irreversible oxidation.
It is another object to provide a secondary alkaline zinc-manganese dioxide cell having an anode current collector exhibiting low internal resistance and wherein the susceptibility of the current collector composition to oxidation is significantly reduced.
It is still another object to provide a secondary alkaline zinc-manganese dioxide cell having an anode current collector having low internal resistance, the current collector composition being modified by inclusion of up to about 11% by weight silicon.
It is a further object to provide a secondary alkaline zinc-manganese dioxide cell which proceeds through substantially only chemically reversible reactions during normal discharge and charge stages of the discharge/charge cycle.